As is known, for reading and writing a hard disk, use is generally made of a suspension that faces a surface of the hard disk when in an operative condition, so as to perform roll and pitch movements and to follow the surface of the hard disk.
It is also known to actuate a head by means of a double actuation stage, wherein a first actuation stage carries out a coarse movement of the head during tracking, and a second actuation stage performs finer adjustment of the head position. To implement the second actuation stage, it has been proposed to use an integrated microactuator of a rotary electrostatic type, interposed between the suspension and the head, to control the position of the head with micrometric accuracy.
An integrated microactuator 1 of an electrostatic type is shown schematically in FIG. 1. For a more detailed description of a structure of an integrated, rotary electrostatic microactuator, see for example U.S. Pat. No. 5,025,346. As shown in FIG. 1, the microactuator 1 comprises an outer stator 2, which is connected when in use to a suspension (not shown), and an inner rotor 4 electrostatically coupled to the stator 2 and supporting a read/write head (not shown).
The rotor 4 comprises a suspended mass 6 with a substantially circular shape, and a plurality of mobile arms 8 extending radially towards the exterior, starting from the suspended mass 6, and identical to one another and angularly equidistant from one another. Each mobile arm 8 supports a plurality of mobile electrodes 10 that extend in a substantially circumferential direction on both sides of the respective mobile arm 8.
The rotor 4 additionally comprises anchorages 14 and resilient suspension elements (shown as springs 12), resiliently connecting the suspended mass 6 to the anchorage regions 14, through which the rotor 4 and the mobile electrodes 10 are biased.
The stator 2 (only a part thereof is shown in full, owing to the symmetry of the structure) comprises a plurality of pairs of first and second fixed arms 20a, 20b arranged alternately to each other and extending radially towards the suspended mass 6, starting from fixed regions 22a, 22b disposed circumferentially around the rotor 4 and intercalated with the anchorage regions 14. The fixed regions 22a are connected to each other, as are the fixed regions 22b, as explained in detail hereinafter. These fixed regions 22a, 22b thus electrically define two nodes, which for simplicity are shown hereinafter as a first node 22a and a second node 22b.
The pairs of fixed arms 20a, 20b are associated with the mobile arms 8, such that a mobile arm 8 of the rotor 4 is arranged between two fixed arms 20a, 20b of each pair. Each fixed arm 20a, 20b also supports a plurality of fixed electrodes 24 extending in a substantially circumferential direction towards a corresponding mobile arm 8. The fixed electrodes 24 are interdigitated with the mobile electrodes 10 of the respective mobile arms 8. In the microactuator 1, the first fixed arms 20a, arranged on a same side of the respective mobile arms 8 (in the example illustrated in FIG. 1, the first fixed arms 20a are located to the right of the mobile arms 8), are all connected to the first node 22a, and are thus all biased to a same first potential. The second fixed arms 20b, arranged on the other side of the respective mobile arms 8 (in the example illustrated in FIG. 1, the second fixed arms 20b are located to the left of the mobile arms 8), are all connected to the second node 22b, and are thus all biased to a same second potential. A capacitive coupling is thereby provided between each fixed electrode 24 and the respective mobile electrode 10. The structure is electrically equivalent to a first plurality of capacitors connected in parallel between the first node 22a and the suspended mass 6, and to a second plurality of capacitors connected in parallel between the suspended mass 6 and the second node 22b.
The microactuator 1 is connected via the nodes 22a, 22b to a drive stage 30 (shown in FIG. 2), the purpose of which is to apply potential differences between the fixed arms 20a, 20b and the respective mobile arm 8, so as to rotate the rotor 4 with respect to the stator 2. In particular, due to capacitive coupling between each mobile arm 8 and the respective pair of fixed arms 20a, 20b, the suspended mass 6 is subjected to a transverse force that is proportional to the number of pairs of fixed arms 20a, 20b and mobile arms 24 associated with each other. This force tends to space the mobile arm 8 from a fixed arm 20a, 20b having a lower potential difference, and to draw the mobile arm 8 closer to a fixed arm 20b, 20a having a higher potential difference. Thus a rotation of the suspended mass 6 is caused to consequently actuate the read/write head.
In prototypes of microactuators proposed hitherto, the nodes 22a, 22b are also used to obtain data relating to the relative positions of the rotor 4 and the stator 2. The nodes 22a, 22b are thus simultaneously drive nodes and measure nodes. A position signal obtained thereby is then used in a feedback loop to carry out adjustment of the position of the read/write head. Therefore, it is possible to increase a mechanical band of a microactuator-head system and to read data recorded on increasingly narrow and dense tracks of the hard disk. This solution is described for example in an article entitled "Magnetic Recording Head Positioning at Very High Track Densities Using a Microactuator-Based Two-Stage Servo System" by Long-Sheng Fan, Hal H. Ottensen, Timothy C. Reiley and Roger W. Wood, IEEE Transaction on Industrial Electronics, vol. 42, no. 3, June 1995.
However, since voltages generated by the drive stage 30 have relatively high amplitudes of approximately 80 V, and on the other hand a stage downstream (shown as a measure stage 32 in FIG. 2) that processes the obtained position data operates with much lower voltages of approximately 5 V, it is often necessary to arrange an uncoupling structure 34 between the nodes 22a, 22b and the measure stage 32 to obtain required displacements of the rotor 4.
FIG. 2 shows an electrical equivalent of the microactuator 1, comprising two variable capacitors 40, 42 arranged in series and representing respective capacitive coupling between the electrodes 24, 10 of the first fixed arms 20a and the mobile arm 8, respectively, and between the electrodes 10, 24 of the mobile arm 8 and the second fixed arms 20b, respectively. In particular, in FIG. 2, an intermediate node 6 between the two variable capacitors 40, 42 represents the suspended mass 6 of the rotor 4.
The uncoupling structure 34 comprises two disconnection capacitors 44, 46. In particular, the disconnection capacitor 44 is arranged between the first node 22a and a first input of the measure stage 32 (represented as an operational amplifier), and the disconnection capacitor 46 is arranged between the second node 22a and a second input of the measure stage 32. In practice, the disconnection capacitors 44, 46 are arranged on a path of a signal containing data related to a position of the rotor 4 with respect to the stator 2. In these proposed microactuators, the uncoupling capacitors 44, 46 are formed through two metal layers at different levels, so as to both prevent distortion of the signal containing the data related the position of the rotor 4 and to withstand high ohmic drops present in the structure.
The microactuators proposed hitherto have many disadvantages. In particular, the uncoupling capacitors 44, 46 form, with the capacitors 40, 42, a capacitive divider that causes signal attenuation, thereby reducing resolution of measurements made by the measure stage 32. In addition, parasitic capacitances associated with metal layers that form the uncoupling capacitors 44, 46 further reduce the signal supplied to the measure stage 32.
Also, the microactuators proposed hitherto occupy a very large area, because of both the specific low capacitance of the metal layers and because of signal-processing circuits that have a lower background noise level than the amplitude of the data signals (of approximately 100 .mu.V). The large area occupied consequently causes an increase in weight which the suspension supporting the read/write head must bear. This lowers a mechanical resonance frequency and gives rise to an increase in volume, causing flight problems of the read/write head due to aerodynamics deterioration and increased inertia of the microactuator-head unit.